Luminescent material



Patented Dec; 1939 signor to Radio Corporation of poration of Delaware America, a cor No Drawing. Original application March 19,

1937, Serial No. 131,862.

Divided and this application August 28, 1937, Serial No. 161,529

4 Claims. (01. 250-81) This application is a divisional of application Serial No. 131,862, filed March 19, 1937, and entitled Luminescent material, which issued August 29, 1939, as Patent No. 2,171,145.

This invention relates to a process for synthesizing luminescent materials and, in particular, is directed to producing luminescent materials of improved properties and with the further advantage that the spectral luminosity of the resultant material may be predetermined at the time of manufacture.

Luminescent materials, particularly those adapted to become excited under the influence of electric bombardment such as cathode ray beam and tubes used for television, oscillographs and allied uses have been known for some time by the commonly used luminescent materials known to the workers in the art, and always had the drawback of lacking consistent reproduction of their characteristic properties within close limits. For example, batches of luminescent material obtained at difi'erent times hithertofore from natural sources or from those synthesized in laboratories or factories would show different spectral emission characteristics due to an inability to produce materials having uniform charac- I distribution. It will be readily appreciated,

therefore, that this new luminescent material has many qualities which make it extremely desirable for use in cathode ray tubes, for example.

My present invention relates to a process for producing and/or preparing a luminescent mate-. rial possessing the above characteristics. In addition, the process of manufacture results in the production of a luminescent material wherein the very valuable property of controlled spectral emission is established by the process of.manufacture. In the past, luminescent materials have had a characteristic spectral distribution which was solely a function of the elements making up the material and in this respect the color of the emitted luminosity under cathode ray bombardment was fixed.

By my invention 1 have developed a luminescent material wherein it ispossible to control the color of the emitted light over a wide spectral range, by merely changing the proportions of the elements entering into the composition of the final material and/or the final heating temperavention to provide a luminescent material whose spectral emission may be changed from one end of the spectrum to the other through all the intermediate positions thereof, by controlling the temperature and duration of the-heating of the luminescent product during its preparation.

A further object of my invention is to shift the emission spectrum from the violet end toward longer wave lengths up to and including the red end by isomorphic mutual replacements in the crystal lattice of the luminescent material.

Another object of my invention is to prepare a material which will emit light of high intensity under cathode ray bombardment.

A still further object of my invention is to provide a process for synthesizing a luminescent material with invariant spectral distribution with regard to length of time of cathode ray bombardment. r

Another object of my invention is to prepare a material which will produce the effect of emitting white light under high intensity cathode ray bombardment. This is accomplished by providing a luminescent material whose emission spectrum characteristic is such that when taken in conjunction with the spectral response characteristic of the human eye, the over-all characteristic is one which provides sumcient energy at all visible wave lengths to give the visible sensation of white. r

A still further object of my invention is to provide a process whereby improved luminescent materials may be prepared with minimum expense but, nevertheless possess the advantage of uniformly superior response characteristics.

Other objectsand advantages of my invention will be immediately apparent to those skilled in the art upon reading the following description of my invention. a

The luminescent material resulting from my invention is a crystalline structure comprising zinc ortho-silicate and zirconium ortho-silicate. activated by manganese, and may incorporate an excess of silicon dioxide held in the crystalline formation of the material. It should. be clearly understood that this is not a mixture of zinc ortho-silicate and zirconium ortho-silicate in the usual chemical sense, but actually is a homogeneous crystal comprising manganese zinc orthosilicate and zirconium ortho-silicate in a single crystal lattice structure or where an excess amount of silicon dioxide is used, as will be described later, then the zinc ortho-silicate, zirconium ortho-silicate, manganese and silicon dioxide are held in a single lattic structure. In this material the ratio between the zinc orthosilicate and the zirconium o'rtho-silicate may be varied from 10,000 to 1 to 1 to 10,000. Thus a formula which is descriptive of a luminescent 20 material possessing the above outlined characteristics, may be expressed as follows:

:r(ZnzSiO4)'J(ZrSi04) Mn In this formula the variable a: and y may be such that :r/y may be varied from 10,000/1 to 1/10,000. This formula is descriptive of where the amount of silicon dioxide is such to give the exact ortho-' silicate proportions. However, where these proportions are not followed and an excess or insufficient amount of silicon dioxide is used, then the following formula is descriptive of the luminescent material:

Here the ratio between u, e and 10 may be varied over extremely wide limits such as, for example, as indicated above.

The composition may also be written as indicating that the manganese activator may be inthe form of an oxide, and to further indicate that the formula does not mean a chemical composition in the usual sense which chemical formulas indicate. -It will be appreciated, of course, that the colon indicates that the substances following the colon are present in very small quantions to 100% or greater than the ortho-propor--- tions. In practice it has been determined that the amount of silicon .dioxide'may be varied from a lower limit of approximately 20% to an. upper limitof approximately 500% of the amount which would be necessary, calculated on the basisof ortho-proportions. The amount of manganese may likewise be varied within wide limits,for example, between values of 0.1 gram-moles to 0.00001 gram-moles with respect to the total cation gram-molecular weight. While the amount of manganese may be varied between these wide limits so as to control, in part, the spectral response, it is found that when the amount of manganese is approximately 0.006 gram-moles, the

maximum efficiency in conversion of energy to light by the luminescent material is obtained.

Should this proportion of manganese not give the required spectral response, the spectral response may be shifted by other means such as the heat treatment or charrging the proportions of the zinc, zirconium or silicon dioxides.

If it is desired, the spectral response may be shifted or controlled by substitution of titanium, hafnium and/or thorium in whole or in part, for the zirconium. Thus it appears that a metal in the first sub-group of the fourth vertical group of the periodic system having an ionic radius lying between 0.35. and 0.7A may be used.

.It may be pointed out at this point that beryllium may be substituted in part or in whole for zinc in the above formula in accordance with the method and means described in my co-pending application entitled Luminescent materials, Serial No. 66,453, filed September 29, 1936. In fact, any metal in the second vertical group of the periodic system, whose oxide'does not decompose below 700 C. can be substituted" in part or in whole for zinc. The metals thus falling within this group, in addition or beryllium, would be magnesium, cadmium, calcium, strontium and barium.

Likewise, germanium may be substituted for silicon in part or in whole, as explained and described in my co-pending application entitled Process for synthesizing luminescent materials, Serial No. 707,866, filed January 23, 1934.

The steps in the process of mixing, precipitating, heating, and finally grinding the luminescent material as in. a ball-mill, for example. have already been described in detail by my copending application Serial No. 707,866, filed January 23, 1934, in which I have described the preparation of a manganese activated ortho-silicate. The essential differences between the preparation of this material and that described in the above mentioned copending application is that in the preparation of my manganese activated zinc zirconium ortho-silicate material, exceedingly pure zinc nitrate and zirconium nitrate solutions are added to very finely divided silicon dioxide in suspension as, for example, colloidal silicon dioxide, in proper portion, as specified by the above mentioned conditions to be met with regard to a: and 1 1,.in contra-distinction to the use only of zinc nitrate. The mixture is heated and stirred and when brought to a boil, there is added very slowly and carefully with plenty of agitation saturated ammonium carbonate solution of great purity and in sufficient quantity to precipitate the metallic salts as carbonates on the finely divided silicon dioxide. The contents of the beaker are then evaporated with stirring to dryness and then heated by any appropriate manner to red heat. The contents are then allowed to cool and are ground and mixed with a quartz rod. The resultant product is then placed in a quartz crucible and a suitable quantity of manganese nitrate solution of the greatest purity obtainable is added,

the exact quantity ranging between 0.1 and 0.00001 gram-moles with respect to the .total cation gram-molecular weight depending on the actual spectral emission distribution desired.

Quartz distilled water is then added to the amount to make the mixture thoroughly wet.

The crucible is then heated and the contents thereof stirred until brought to a boil, whereupon concentrated ammonium carbonate is added to precipitate the manganese, in the form of manpreviously precipipresent and co-precipitated with the zinc and zirconium right at the esinninawhioh step is desirable in commercial production when the optimum activator concentration has been determined and is known. The separate or extra activation step first described is a convenience to make variation of the activator easier. When the optimum value has been determined, all the components may be precipitated at once and so save the extra step in the process.

The contents of 'this crucible are then evaporated with stirring to dryness and upon cooling the contents are ground with a quartz rod. The contents are then transferred to a covered platinum crucible and heatedto between 700 and 1600 C. in an electric furnace, for example. It

has been found by experience that the optimum temperature lies between l100-l500 C. and the heating time is preferably on the order of an hour. The final product is alightly fritted powderv or a fused mass which gives intense cathode luminescence, whose color depends upon the actual ratio between the zinc ortho-silicate and zirconium orthc-silicate. For a ratio of unity between these two materials and under high intensity electron bombardment, the emitted light appears white. It, will be appreciated, of course, that by changing the ratio of the zinc ortho-silicate to the zirconium ortho-silicate, the color may be shifted from the normal green blue of pure manganese activated zinc ortho-silicate toward the red end of the spectrum to give a more pleasing color. Other changes in the spectral emission spectrum may be had by changing the amount of manganese or by controlling the temperature to which the product is heated, or by controlling the length of time that the material is heated. v

As was pointed out above, substitution of beryllium in whole or in part for zinc, germanium in whole or in part for silicon, or titanium, hafnium and/or thorium in whole or in part for zirconium, gives further means of changing the spectral emission characteristic of the resultant luminescent product.

I may further modify and/or control the emission spectrum characteristic by means of hightemperature quenching as described in my copending application, Serial No. 59,883, flled January 20, 1936, entitled Luminescent materials."

By means of hightemperature quenching the lattice structure of my improved crystalline material may be expanded with a consequent shift in the emission spectrum toward the longerwave lengths. As a result of this, additional advantages accrue to my materials and methods. since it is possible to utilize a single prepared batch of material for purposes requiring diflerent spectral emission characteristics by the additional step of high-temperature quenching.

In this process it will be noted that no halides and, in particular, no fluorides are used in the process of preparing the composition. Consequently, the necessity for providing platinum ware forusein preparation or mylmproved luminescent material is unnecessary. Therefore, it is readily seen that the cost of preparing such'materials is materially reduced. It will be apparent from the foregoing that this composition of material described by the present invention, in addition to the property of having controlled spectral range,

possesses the other desirable features of manganese activated zinc ortho-silicate, which have been enumerated in my co-pending application, referred to above, among which are high efllcient emitting light of high intensity under cathode ray bombardment possessing an invariant spectral distribution with regard to the length of time of cathode ray bombardment, which has cheapness of preparation,controllable spectral emission, and precise spectral emission characteristics controlled by temperature and heating process.

Consequently, my improved process and material have made possible the production of television pictures of improved quality by providing a material which can be coated on the end wall of a cathode ray tube upon which the electrooptical representation is produced, since the color of the resulting luminescence may be controlled to give a white color of high intensity. It will be readily appreciated that reconstructed images on the end wall of the cathode ray tubes in white color are preferable to the usual bluish green image, since people are accustomed to black and white pictures, and my new and improved materials affords a practical realization of this ideal,

whereas the bluish green images produced by prior known phosphors are wide of the mark in this respect. I If it is desired to accentuate any one part of the spectrum, the manganese activated zinc zirconium silicate spectral emission may be peaked in almost any region either by heat treatment or by choosing the suitable proportions between zirconium and zinc, or the activator manganese. If itis desired to increase the secondary emission of this material. the manganese activated zinc zirconium silicate which I have described, may be mixed with small amounts of barium, strontium, calcium, caesium, rubidium, lanthanum, cerium, thorium, any of their compounds, or other elements or their compounds, which have large ionic or atomic radii. In-certain applications of luminescent material, it is desirable to have high secondary emission-and by mixing small amounts of the above identified materials, this desirable feature can be readily obtained.

Having described my invention, what I claim 1 4. A luminescent material consisting of manganese activated beryllium thorium germanate. HUMBOLDI W. 

